221blastoderm margin is normal in hec eggs, underscoring the independence of these two
transport systems. Sybu encodes a potential cargo linking protein for Kif5b, suggest-
ing a role in microtubule transport of dorsal determinants vegetally. Grip2a encodes a
scaffolding protein important for subcellular localization in mammalian neurons.
Both sybu and grip2a mRNAs localize to the vegetal cortex, along with wnt8a
mRNA (see below), and these RNAs all undergo an off-center “shift,” mirroring that
of the microtubule array (Nojima et al. 2010 ; Lu et al. 2011 ; Ge et al. 2014 ). Sybu
protein fails to localize to the prospective dorsal side in nocodazole-treated eggs,
suggesting it is trafficked by microtubules. The exact role of Grip2a is not known
but it may recruit protein complexes to vesicles or organelles that attach to and help
align microtubules. Interestingly, grip2 mRNA is localized in Xenopus, but follows
a germ plasm-like pattern and is not thought to play a role in axis formation
(Tarbashevich et al. 2007 ). Similarly, sybu is localized to the germ plasm in Xenopus
and may play an undefined role in axis formation, possibly in transport or in Wnt
activation (Colozza and De Robertis 2014 ).
In zebrafish, maternal loss-of-function mutants have implicated kif5ba in vegetal
microtubule formation and axis formation (Campbell et al. 2015 ), although its role
is complex. Organized vegetal microtubules fail to form and wnt8a does not shift
dorsally. However, grip2a asymmetric translocation still occurs and sybu RNA is
not maintained vegetally (Campbell et al. 2015 ). It is unclear to what extent these
phenotypes reflect roles for Kif5ba in localizing components vegetally during
oogenesis or more acute roles during microtubule organization and transport.
6.2.3 Asymmetry in Early Amniote Embryos
Initial axis formation in fish and frogs occurs in the fertilized egg; the dorsal deter-
minants are either inherited directly by dorsal cells (frogs, primitive fish) or trans-
mitted from the uncleaved yolk cell to peripheral dYSL and overlying dorsal
marginal blastomeres (teleost fish). The axis in amniotes (birds, reptiles and mam-
mals) relies on mainly on reciprocal interactions between upper embryonic and
lower extraembryonic tissue layers (epiblast and hypoblast, respectively) and asym-
metric cell movements, with only hints that early asymmetry in the egg or early
embryo are involved. Additionally, the links to localized activation of growth factor
signaling pathways are much less clear.
6.2.3.1 The Role of Gravity in Axis Formation in Sauropsids
Classic experiments in the chick have suggested that axis specification occurs in
response to gravity as the egg rotates as it passes through the oviduct (Fig. 6.5). Axis
formation in reptile embryos is thought to occur in a similar fashion, although is less
thoroughly studied in this regard. Bird and reptile embryos are both highly poly-
spermic, making it unlikely that sperm entry plays a role in axis formation
6 Vertebrate Axial Patterning: From Egg to Asymmetry